The microprocessor has proven to be more difficult to cool with each new generation of technology. The trend of more transistors and higher switching speeds drives the power dissipation levels for processors up, while the cost of manufacturing silicon chips and the need to maintain or decrease signal path lengths keeps the area emitting that power relatively small. One parameter which quantifies the difficulty of cooling a the back side of a processor is the required heat transfer coefficient, which is thermal power per unit area per available temperature difference. The heat transfer coefficient can be expressed as watts per (degrees Centigrade square centimeters). For example, the required heat transfer coefficient required for the back side contact to a 300 MHz Pentium II processor in 1997 is approximately 0.7 (watt/.degree. C. cm.sup.2). The Intel technology roadmap says that in the year 2006 a processor will release 200 watts of heat into a 15 millimeter square chip with a junction temperature of 50.degree. C. above ambient, so that the required heat transfer coefficient at the chip will be 1.8 (watt/.degree. C. cm.sup.2).
Two broad classes of heat transfer devices are currently in use. One class conducts the heat from the heat source to a heat sink through a relatively stationary solid material--a passive thermal spreader. Most microprocessor coolers today use this design; an example is a spreader plate with attached fins, plus a fan to circulate air through the fins. They are typically relatively simple, compact, reliable, and inexpensive. Passive spreader devices are characterized by relatively lower heat transfer coefficients; the heat transfer coefficients achieved by these devices are typically less than 1.2 (watt/.degree. C. cm.sup.2).
Another broad class of heat transfer devices uses an intermediate fluid to transport the heat from the heat source to the heat sink--a thermal convector. An automobile engine today is typically cooled this way; heat from the block is conducted into water, which is pumped into a radiator, where the heat is then conducted into a forced air flow. They are typically relatively complex and expensive compared to the first class of heat transfer devices. These heat transfer devices are characterized by relatively higher heat transfer coefficients; the heat transfer coefficients achieved by these devices can be as high as 10 (watt/.degree. C. cm.sup.2).
The heat transfer coefficient requirements of microprocessors are crossing the gray transition between the two classes. Other devices such as some laser bar arrays and power transistors will have or already have crossed that transition. There is an economic impetus to develop a different class of heat transfer devices which have the high heat transfer coefficients characteristics of the second class while retaining as many of the positive characteristics of the first class as possible. This different class of heat transfer devices might have utility beyond cooling electronics devices, including other applications such as aerospace, manufacturing processes, chemical engineering, power generation, and material processing (such as tempering).